Shaped polyacrylonitrile structures



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The invention -relates to the manufacture of shaped structures from polymers and copolymers of acrylonitrile.

In my copending application Ser. No. 311,530, filed September 25, 1952, I have described a process of preparing shaped structures from polymers or copolymers of acrylonitrile which comprises dissolving an acrylonitrile polymer containing at least '80 percent of acrylonitrile at temperatures below 480 C., preferably between and 50 C., in nitric acid having a concentration of 46 to 68 percent, preferably 52 to 65 percent by weight of HNO3. The polymer can be recovered from these solutions as a shaped structure, e.g. in the form of filaments by dry or wet Spinning methods, where the nitric acid is evaporated or diluted with non-solvents.

When I investigated the use of said solutions for the wet Spinning process, I obtained very unexpected results. I found that said solutions may be spun to filaments when they are extruded through a spinneret into a coagulating bath, consisting essentially of water and containing nitric acid, in an amount adjusted to the spinning speed and to the rate of the Water feed to the bath. However, it was also noted that the properties of the coagulated products were strongly influenced by the temperature of the coagulating bath. If the coagulation Was carried out below the critical temperature, which term will be explained below, high lustrous silk-like filaments Were obtained as expected. But when the temperature of the coagulating bath was raised to a temperature within the critical range, such high lustrous filaments were no longer obtained; according to the temperature raise above the critical point, which was maintained in the coagulating bath, the obtained filaments were less lustrous or Without lustre and assumed a dull to white aspect,'which is due to a porous structure.

The critical temperature range is to a certain eXtent a function of the composition of the polymer but is always between 15 and 70 C. If a solution of polyacrylonitrile in nitric acid is spun, and if the coagulating bath has a temperature below l C., the filaments are lustrous and have a silky character. If, however, the temperature is raised above 15 C., the filaments assume with rising temperature more and more a dull and white aspect. Of course, the transitions from a lustrous and clear filament to a dull and white filament, which are produced by raising the temperature of the coagulating bath, are not abrupt but gradual, and the transition temperatures Vary as a function of Vthe composition and type of filament. Some polymers, for instance, show the described e'fiect already at bath temperatures of 15 to 20 C. The lower temperature limit, however, is generally above 20 C., i.e'. from this temperature on the effect .becomes visible for the naked eye. The upper temperature limit is defined by the spinnability of the filaments.

It was to be assumed that a shaped porous structure would be unsuitable for the subsequent stretching operation, which is necessary to improve the mechanical properties so as to produce a commercially useful filament. However, the opposite is the case.` By extruding the s aterit structure, which is still filled with solution.

nitric acid solution into warm water a filament is obtained the interior of which presents over the whole length of the filament a very fine regular porous structure; this makes it possible to spin and Stretch the filaments without difliculties. This behavior appears to be typical for nitric acid solutions. So far, it has not been possible to accomplish similar efi'ects with other solvents. As already stated, the upper temperature limit is controlled by the spinnability and elongation. As distinctly shown in the preparation of films on glass plates, the size of the cavities increases with increasing temperature until the Spinning of a fine filament becomes impossible, Though for coarser deniers it is possible to Work still 'at 70 C., the matting effect is already at 30 to 40 C. a maximum. The critical temperature range in its broadest aspect may, therefore, be defined as lying between 15 to 70 C.; the optimum range is between 25 and 50 C.

It was found that a filament produced within the critical temperature range has, for the same titer, a larger diameter than a filament spun below said range. The cross section of the filaments are usually circular; whereas, for instance, a filament of 3 denier which had been pro- Vduced at temperatures of the coagulating bath between 5 and 15 C., has a diameter of 19 to 20/t, a similar filament produced otherwise under the same conditions but in a coagulating bath having a temperature of 35 to 40 C., had a diameter of 24 to 29,11..

In the accompanying drawing, I show, in comparison, on a very much enlarged scale, microscopic pictures of the cross section of a single fibril of unstretched filaments of the same titer, wherein:

Fig. l is taken from a lustrous filament, wherein the fibrils have a diameter of 19,u, and

Fig. 2 is a corresponding cross section taken from a dull filament according to the invention, wherein the fibrils are larger and have, due to the enclosed air bubbles, a diameter of 24/4.

The microscopic picture of stretched filaments shows the lustrous filaments clear and transparent, whereas the mat filaments appear even in thin cross sections opaque and dull, though inhomogenities proper, e.g. air bubbles, cannot be made out. But if the filament is examined in unstretched condition, i.e. in the state as it leaves the coagulating bath after rinsing with water and drying, innumerable bubbles can be detected which are just visible. Otherwise, the filament in this unfinished, i.e. unstretched state constitutes the same unstable structure as the lustrous filament. Surprisingly, however, the mechanical properties and the strength of the mat porous filaments in the stretched state are at least not inferior to those of the transparent lustrous filaments.

It is not yet fully understood how the porous structure is originated solely by raising the temperature in the coagulating bath by a few degrees. It seems that the rate of coagulation has some influence. At higher temperanozzle with a diameter of e.g. '2-5n, solidifies to a tubular Presently, the acid diifuses out through the walls of the tube, water diifuses in instead, and lon drying, the water is replaced by air. However, in the decisive stages the outer wall has become, by the higher coagulating temperature, already so rigid that it can no longer follow the contraction of the filamentary mass, and the inner filamentary mass is no longer suflicient to fill out the tube, Which results in the formation of vacuoles. At lower temperatures of the coagulating bath, the entire filamentary mass, including the outer portions, remain in a swollen elastic state,

which allows the whole mass to contract conforming to the loss of solvent and water; no cavities are formed.

My ,novel method allows of producing instead of lustrous filaments of dense structure dull filaments of porous structure merely by modifying the temperature of the coagulating bath. The degree of porosity can be controlled by adjusting the rise of temperature.

The invention offers the following advantages:

(1) It is possible by the mere adjustment of the temperature of the coagulating bath to a predetermined degree, without additions, to produce on filaments delustering effects or a predetermined degree of whiteness.

(2.) The stretched mat or delustered filaments show no reduced strength in comparison with the high lustrous filaments.

(3) The mat filaments are produced more economically than the lustrous filaments. For the same weight, a larger amount by volume of dull filaments is obtained as the dull filaments have a smaller apparent specific gravity.

(4) The mat filaments produced according to the invention have a better heat retaining capacity than lustrous filaments.

Polymers or copolymers of acrylonitrile can be used for carrying out the novel process; if copolymers are used, they must contain at least 80 percent by weight of acrylonitrile, the rest being another polymerizable compound such as vinyl acetate, arcylic or methacrylic acid or esters or amides thereof.

The nitric acid content of the coagulating bath depends on the rate of feed and is between 1 and 40 percent by weight. i

The following examples illustrate the method and results obtained in controlling the temperature of the coagulating bath, whereby the first example is carried out at a temperature below the critical temperature and the following examples above the critical temperature.

Example I 100 g. of polyacrylonitrile having an average molecular Weight of 60,000 and l g. of urea are dissolved at 20 C. in 700` g. of 59%l nitric acid. The solution is filtered through a Nichrome Wire net with 50,1l meshes and stored at a temperature of to C. At this temperature, the solution is stable for at least 100 hours, i.e. after said time the polymer can be quantitatively recovered with substantially the same nitrogen content by precipitation with water. The solution has a viscosity of 450 poise at C. It is extruded into a coagulating bath at a rate of 36 cc. per hour through a spinneret having 10 holes of .1 mm. diameter. The coagulating bath has a temperature of 10 to 15 C. and consists of 10% nitric acid. The filament travels through the bath for a distance of 150 mm. and is then rinsed With distilled water in countercurrent in a washing trough having a length of 500 mm. The filament is then wound on a take-up bobbin revolving at a circurnferential speed of 8 m./min. From this bobbin, the filament passes through a water bath of 90 to 95 C., which contains 0.l% of phosphoric acid, 02% of polyvinyl alcohol, and 02% of glycerol; in this bath, the filament gives olf the main amount of its Water content. Subsequently, the filament passes over a guide roller into a drying chamber, where it is dried with air of about 100 C., and then over a bobbin heated to 150 C. and revolving at a circumferential speed of 10'm./min to the collecting bobbin, which revolves at a circumferential speed at 30 m./rnin. In this manner, a 30 denier fiber, consisting of 10 filaments, is obtained, which has a tensile strength of 120 to 150 g. The fiber is lustrous and colorless; its cross-section is Circular and has a diameter of 19 to ZOM.

Example 2 The procedure Was exactly the same as in the first example with the sole difference that the temperature in the coagulating bath Was maintained at 35 to 40 C. instead of at 10' to 15 C. There was also obtained a 30 denier fiber which consisted of 10'individual`filaments and. had a tensilestrength of 120 to 150 g. However,

4 the fiber had dull lustre and a snow White color. The cross section was Circular, and the diameter was 22 to 2411..

Example 3 As in Example 1, a solution of g. of polyacrylonitrile having an average molecular weight of 65000 and 800 g. of 59% nitric acid is prepared at 20 C. The solution has a viscosity of 400 poisc at 15 C. It is spun as described in Example l, with the following modifications. The spinneret has 10 holes of .2 mm. diameter and is fed With 40 ccm. of solution per hour. The spinning speed is 3 m./min at the spi'nneret and 30 Iin/min on the take-up bobbin. The temperature in the coagulating bath is maintained at 40 to 45 C. The fiber has a total titre of 30 d. and a tensile strength of g. It is dull and white. The cross section of the fiber is circular and has a diameter of 25 to 28.

Example 4 Acrylonitfile and Vinyl acetate are polymerized in a known manner to a copolymer containing 5 percent of Vinyl acetate and having a molecular weight of 70,000. The polymer is connninuted to a particle size of less than a diameter. 100 parts of this copolymer are ground to a suspension with 350 parts of a nitric acid containing 53 percent by weight of HNO3. This suspension is degassed at reduced pressure. Then 1 g. of urea and 350 parts of 65% nitric acid are introduced with stirring. Within a few minutes, the suspension changes to a clear solution. Said solution is filtered through a finely porous filter of a polyvinyl chloride fabric and then degassed in vacuo. All these operations are carried out at about 15 C. An almost clear solution is obtained, which has at 20 C. a viscosity of about 350 poise. This solution is spun, as described in Example 1, in a coagulating bath, which consists of 30% nitric acid. The temperature in the coagulating bath is kept at 2 to 5 C., and a clear lustrous tilament is obtained. Subsequently, the temperature in the coagulating bath is slo'wly raised. It can be observed how the aspect of the filament changes with rising temperature. First, it turns milky transparent, and from about 20 C. up it assumes a dull white aspect. The volume of the filamentary mass produced p'er unit of time increases with increasing temperature.

The Spinning process can be carried out up to' temperatures of 35 to 40 C. in the coagulati'ng bath' Without disturbances; at a further increase of the temperature of the coagulating bath, however, the filament easily tends to break before it is sufliciently strengthen'ed by' stretching.

The following table shows the'properties of the filament as a function of the temperature of the coagulating bath; line A indicates the tensile strength in grams'perv 1 denier, line B the diameter of the filament in microns, and line C the aspect of the filament.

Temperature 5 C 15 C. 30 C.

A 4.5- 4.6.- 4.8. B 19 22 28. C clear, lustrous-.- millty, transparent.- dull, white.

I'n the same manner copolymers of acrylonitrile and smaller amounts of methacrylamide, rnethacrylonitrile, dimethylarhinoethyl methacrylate, vinyl pyridine, Vinylirnidazol, and the like can be processed with very similarthe coagulating bath, it will be understood that the process is susceptible of various modifications. The Spinning in the Water bath and the subsequent processing of the filaments may be carried outin various ways. For instance,

after the solutions have been spun into Water to obtain lustrous or according to the invention dull filaments, the

filaments can be Washed first With Water, and subsequently they can be treated With hot water of more than 70 C. to remove the greater part of the absorbed Water; the Washed filaments are then heated to about 70 to 100 C., for instance in Water, and stretched by at least about 30 percent, and these prestretched filaments are subsequently stretched again at temperatures above 100 C. by to 250%. Or the swollen filaments can be heated to 70 to 100 C., for instance by passing them over a heated support, to remove the greater part of the absorbed water in liquid form, and the recovered hot water may, if desired, be used for treating the freshly spun filaments at temperatur-es of 70 to 100" C. The after-stretching can also be carried out in a water bath Which contains traces of a strong acid, and after the wet stretching, the filament may be subjected to a finishing treatment by which small amounts of a strong acid and a lubricating agent are applied.

What I claim is:

1. A delustered filament composed of an acrylonitrile polymer selected from the group consisting of polyacrylonitrile and copolymers containing at least 80 percent by weight of acrylonitrile monomer units, the balance being polymers of compounds copolymerizable With acrylonitrile, said polymer being soluble in nitric acid, the interior of said filament being honeycombed With a great number of small cavities, said filament having throughout a dull to White appearance and being stretchable without impairment of its mechanical and strength properties to the same extent as a corresponding transparent lustrous filament of the same composition.

2. A delustered stretched filamentV composed of an acrylonitrile polymer selected from the group consisting of polyacrylonitrile and copolymers containing at least 80 percent by weight of acrylonitrile monomer units, the balance being polymers of compounds copolymerizable with acrylonitrile, said polymer being soluble in nitric acid, the interior of said filament being honeycombed with a great number of small cavities, said filament having throughout its cross-section a dull to white appearance, the mechanical and strength properties of said delustered filament being at least the same as those of, and the heat retaining capacity of said delustered filament being considerably improved over that of a corresponding transparent lustrous filament of the same composition. 3. A delustered filament composed of an acrylonitrile polymer selected from the group consisting of polyacrylonitrile and copolymers containing at least percent by weight of acrylonitrile monomer units, the balance being polymers of compounds selected from the group consisting of vinyl acetate, acrylic acid, methacrylic acid, and esters and amides of said acids, said polymer being Soluble in nitric acid, the interior of said filament being honeycombed with a great number of small cavities, said filament having throughout its cross-section a dull to White appearance and being stretchable Without impairment of its mechanical and strength properties to the same eXtent as a corresponding transparent lustrous filament of the same composition.

4. A delustered filament composed of polyacrylonitrile soluble in nitric acid, the interior of said filament being honeycombed With a great number of small cavities, said filament having throughout its cross-section a dull to White appearance and being stretchable Without impairment of its mechanical and strength properties to the same extent as a corresponding transparent lustrous filament of the same composition.

5. A delustered stretched filament composed of polyacrylonitrile soluble in nitric acid, the interior of said filament being honeycombed With a great number of small cavities, said filament having throughout its crosssection a dull to white appearance, the mechanical and strength properties of said delustered filament being substantially the same as those of, andthe heat retaining capacity of said delustered filament being considerably improved over that of, a corresponding transparent lustrous filament of the same composition.

References Cited in the file of this patent UNITED STATES PATENTS 1,427,330 Rousset Aug. 29, 1922 '2,268,160 Miles Dec. 30, 1941 2,385,890 Spanagel Oct. 2, 1945 2,476,293 Hall July 19, 1949 2,530,962 Hare Nov. 21, 1950 2,531,408 D'Alelio Nov. 28, 1950 2,531,409 D'Alelio Nov. 28, 1950 2,555,300 Carlton et al. June 5, 1951 2,572,931 Helle Oct. 30, 1951 2,579,451 Polson Dec. 18, 1951 2,628,945 Wayne Feb. 17, `1953 

1. A DELUSTERED FILAMENT COMPOSED OF AN ACRYLONITRILE POLYMER SELECTED FROM THE GROUP CONSISTING OF POLYACRYLONITRILE AND COPOLYMERS CONTAINING AT LEAST 80 PERCENT BY WEIGHT OF ACRYLONITRILE MONOMER UNITS, THE BALANCE BEING POLYMERS OF COMPOUNDS COPOLYMERIZABLE WITH ACRYLONITRILE, SAID POLYMER BEING SOLUBLE IN NITRIC ACID, THE INTERIOR OF SAID FILAMENT BEING HONEYCOMBED WITH A GREAT NUMBER OF SMALL CAVITIES, SAID FILAMENT HAVING 